1. Field of the Invention
The invention relates to downhole well testing which is a broad term to designate methods to evaluate subterranean rock layers intersected by a well for their potential to produce hydrocarbons.
2. Description of the Prior Art
Downhole well testing consists in lowering an apparatus or combination of apparatuses in the well in order to hydraulically isolate the layer of interest from the rest of the well and enable that layer to either flow into a chamber that is part of the combination of apparatuses or to flow to surface via suitable pipes that are connected to the apparatuses.
After a wellbore has been drilled through the formation, the various layers of the formation are perforated using perforating guns. Following perforation, testing, such as drillstem testing, is performed. Drillstem testing (DST) is a procedure to determine the productive capacity, pressure, permeability and nature of the reservoir fluids, or extent (or some combination of these characteristics) of a hydrocarbon reservoir in each layer of the formation.
In the field of oil and gas well testing, it is common to encounter wells that traverse more than one separate subterranean hydrocarbon bearing zones which may have similar or different characteristics.
In this event, it is today necessary to perform as many Drill Stem Test (DST) trips in the well as there are layers to be tested. This is a source of considerable non-productive time for a drill stem downhole testing operations.
Currently when several layers that are intersected by a given well are to be tested, a separate downhole test is performed on each layer, sequentially starting from the bottom of the well, using a drillstem testing tool (DST tool) also called a test string. At the end of each test, said test string is removed from the well to enable the layer that was just tested to be hydraulically isolated from the well and the test tools to be reset for the next run of the string in the well.
A typical sequence deployed to test two zones in a given well with a downhole testing system according to the prior art is illustrated in FIGS. 1a to 1f. 
As shown in FIG. 1a, the test string 3 comprising a packer 7, a perforating gun system 9 and a tester valve 13 is run into the well 5 in order to position the perforating gun system 9 adjacent to the lowest layer of interest 1. Packer 7 is set to isolate layer 1 from the well bore 5. The layer 1 is then perforated with the perforating gun 9, as shown on FIG. 1b. Accordingly, the layer material 11 flows into the well bore 5 and inside the test string 3 and is tested. For example, pressure is measured and sampling of layer material is performed via pressure gauges and samplers typically positioned below the tester valve 13. The layer 1 is then killed, packer 7 is unset and the test string 3 is pulled from the well 5. Layer 1 is isolated from the upper part of the well bore 5 by setting a plug 15 across or above it (FIG. 1c). The test string 3 is reset and the perforating gun 9 is prepared for the test of the following layer 2. As illustrated on FIG. 1d, the test string 3 is run again into the well 5 to test the layer 2. Packer 7 is set to isolate layer 2 from the well bore 5. The layer 2 is perforated with the perforating gun 9 (FIG. 1e). Layer material 17 flows in the well bore 5 and in the test string 3 and is tested. Once again, pressure may be measured and sampling of layer material may be performed via pressure gauges and samplers positioned below the tester valve 13. Layer 2 is then killed, packer 7 is unset and the test string 3 is pulled from the well 5. On FIG. 1f, layer 2 is isolated from the upper part of the well bore 5 by setting a plug 19 across or above it. Successively, all additional layers of the well 5 may be tested in the same way.
In the system as described above, the test string 3 needs to be removed for each layer to be tested, for the test string 3 to be reset and a plug to be set. As a result, the downhole testing of multiple layers in a wellbore may be a lengthy and costly process.
It may take up to several days which may be costly in terms of labor and equipment costs and which delays the completion of a wellbore.
An example of a multizone testing system is disclosed in U.S. Patent Application No. 2006/0207764. This application relates to an assembly enabling a plurality of layers of interest to be sequentially tested. Said assembly comprises a plurality of valves, each being actuatable by dropping a valve-actuating object into the corresponding valve. The valves are successively actuatable to an open state in a predetermined sequence and the different layers are tested or stimulated after actuating corresponding valves to the open state.
The document mentioned above describes a downhole testing system principally related to the stimulation of the layers. Once actuated, the valves cannot be closed. Accordingly, it doesn't provide any flexibility in the testing of the layers.
The system of the present invention solves the above-mentioned problems by providing a testing system which may be used to test several layers within a single trip of the downhole test string in the well and which provides flexibility in the testing of the layers.